1. Technical Field
The present invention relates to an improved method for characterizing an electron beam.
2. Related Art
Freeform fabrication or additive manufacturing is a method for forming three-dimensional articles through successive fusion of chosen parts of powder layers applied to a work plate. A method and apparatus according to this technique is disclosed in U.S. Pat. No. 7,713,454.
Such an apparatus may comprise a work table on which the three-dimensional article is to be formed, a powder dispenser, arranged to lay down a thin layer of powder on the work table for the formation of a powder bed, an energy beam source for delivering an energy beam spot to the powder whereby fusion of the powder takes place, elements for control of the energy beam spot over the powder bed for the formation of a cross section of the three-dimensional article through fusion of parts of the powder bed, and a controlling computer, in which information is stored concerning consecutive cross sections of the three-dimensional article. A three-dimensional article is formed through consecutive fusions of consecutively formed cross sections of powder layers, successively laid down by the powder dispenser.
In order to melt the powder material at specific locations there is a need to have an accurate control of the energy beam, such as deflection speed, position, and shape.
Presently an electron beam is optically calibrated using the glow of the metal. Such calibration method has several drawbacks. Firstly it is time consuming since it takes some time until the metal starts to glow. Secondly it requires a relatively high beam power for starting the metal to glow. Thirdly, the metal which has been glowing may have been locally damaged or at least locally changed its material characteristics. And lastly, the optical equipment used for the calibration may get metallised during the calibration process or in a later process step. Furthermore there is not a straightforward relationship between the glow of the material and the power of the electron beam.
There is thus a need in the art for a simple and efficient method for calibrating and/or verifying the characteristics of an electron beam.